How do I know if Blender is making use of CUDA to render?
If you have properly set up CUDA and selected GPU Compute in the Render Properties panel then Blender should be using your GPU.  (Though I have seen instances where having two different Blender files open separately and both rendering switches the GPU one to CPU mid-render.)

Probably the easiest way to make sure that Blender is utilizing your GPU is to use some sort of monitoring program.

My Favorite is NZXT CAM, it's pretty sleek and works very well.  CAM monitors and gives graphs of CPU and GPU load, temperature, voltage draws, and fan speeds.  It also shows which programs are using the most load.

HW Monitor is another very popular choice, it doesn't have all the options that CAM has though.  It basically just shows you the current values, minimum, and maximum of each spec.